I. Field of the Invention
The present invention relates generally to a machine for hemming sheet metal assemblies.
II. Description of Related Art
There are many previously known hemming machines. Many industries, such as the automotive industry, utilize sheet metal hemming machines to secure two metal panels together. These sheet metal hemming machines typically comprise a base and hemming tooling mounted to the base. A nest is mounted to the base and the nest and tooling are movable relative to each other. The nest, in turn, supports the part to be hemmed.
At least one, and typically two or more, hemming tool sets are laterally slidably mounted to the base and movable between an extended position and a retracted position. In the extended position, the hemming tool overlaps the nest so that the relative vertical displacement between the nest and the hemming tool causes the part to be hemmed to be compressed between the hemming tool and the nest. Typically, a prehem is first formed by a prehem tool to bend an outer lip of the outer sheet metal panel at an angle of approximately 45xc2x0 relative to the plane of the second or inner sheet metal panel while a final hem die retrorsely flattens the sheet metal hem together.
When performing a hemming operation with the previously known hemming machines, the outer lip on the outer panel is typically folded over the outer peripheral edge of the inner panel so that, following the hemming operation, the outer edge of the inner panel is sandwiched in between the outer sheet metal panel and the outer lip on the sheet metal panel. As such, the hem has a thickness equal to twice the thickness of the outer panel plus the thickness of the inner panel and the bending radius is equal to one-half this thickness. When the inner and outer panels are of the same thickness, the radius of the bend for the previously known hemming machines is equal to 1.5 times the thickness of the sheet metal panel.
While many previously known sheet metal hemming machines produce a smooth hem, the relatively large bend radius of the fold line as viewed from the outside in some cases may reflect light in a number of different directions due to the relatively large radius of the hemming bend. This, in turn, creates a visual optical illusion of increasing the gap space between the hem and the adjacent panel. In many industries, and particularly the automotive industry, minimization of the gap space between adjacent panels is highly desirable and improves the quality perception by the customer. Consequently, these previously known hemming machines disadvantageously increase the visual illusion of the gap space between adjacent panels.
A still further disadvantage of these previously known hemming machines is that the hem is oftentimes subjected to over compression during the hemming operation. Such over compression results in xe2x80x9ctransparencyxe2x80x9d or xe2x80x9cwitnessxe2x80x9d marks, rollback, outside rope and other hemming defects.
The present invention provides a hemming machine which overcomes all of the above-mentioned disadvantages of the previously known devices.
In brief, the present invention is designed to hem an outer and inner sheet metal panel together. The outer sheet metal panel includes an outer lip which lies in a transverse plane with respect to the plane of the inner panel. The lip extends from a bend line in the outer panel to a free edge of the outer panel. The inner panel also includes an outer edge which is spaced inwardly from the bend line in the outer panel by a few millimeters and typically two to three millimeters prior to the prehem operation.
The hemming machine comprises a base and a nest which is vertically slidably mounted relative to the base. The nest, furthermore, is adapted to receive and support the sheet metal assembly to be hemmed.
A final hemming tool is mounted to the base and this tool includes a first generally planar section which overlies the outer edge of the inner panel as well as a portion of the lip on the outer panel. The final hemming tool further includes a second curvilinear section formed on a radius of between three and 100 millimeters adjacent the first planar section. The second curvilinear section is oriented such that its tangent intersects the plane of the first planar section at an angle xcex1 where 0xc2x0 less than xcex1 less than 20xc2x0. The second curvilinear section overlies the remainder of the lip and extends to the bend line in the outer panel.
Conventional means are provided for moving the nest relative to the base. In doing so, the sheet metal assembly compresses against the final hemming tool and sandwiches the outer edge of the inner panel between the outer panel and the lip of the outer panel. However, unlike the previously known hemming devices, small angle, i.e. less than 20xc2x0, of intersection of the curvilinear section with the first planar section of the final hem tool creates a relatively small bending of the lip over the inner panel edge and relatively sharp radius bend along the bend line in the outer panel so that the total thickness of the final hem at the bend line is, at most, twice the thickness of the outer sheet metal panel. This relatively small thickness hem at the outer edge in turn has little or no adverse effect on the visual gap space between the hem and the adjacent panel.
The final hemming tool optionally includes a third planar section which is aligned with the nest and is positioned outwardly from the bend line on the outer sheet metal panel. This third section lies in a plane perpendicular to the direction of travel between the nest and the hemming tooling and is dimensioned to flatly abut against a mating surface on the nest during the hemming operation and thus limit the compression of the inner and outer sheet metal panels between the final hemming tool and the nest without any dispersion introduced by the compliance of the hemming machine. This, in turn, minimizes hemming defects, such as transparency marks, rollback and outside rope, that can result from over compression of the sheet metal assembly between the nest and hemming tool.